# Tag Info

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Let me attempt a more "popular science" answer (Ron please be gentle with me!). In GR a geodesic is the path followed by a freely moving object. There's nothing especially complex about this; if you throw a stone (in a vacuum to avoid air resistance) it follows a geodesic. If the universe is simply connected you'd expect to be able to get anywhere and back ...

9

This issue is largely settled today--- you cannot go to another disconnected universe, but you are either trapped in the black hole, or reemerge in this universe. The reason is the no-information loss property. If material could go between disconnected universes, information about the state of this material would be permanently lost to the other universe. ...

9

You first have to understand what a "white hole" is. It's the time reverse of a black hole. It was rightly pointed out in previous answers that white holes violate the second law of thermodynamics. Now, like anything in thermodynamics, this makes them unlikely but not impossible (unlikely here usually means unlikely even in an astronomical number of ...

5

No. It's only a fictional object that agrees with einstein's formulas. But there is no evidence that that objects exists or had existed.

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It's not exactly a duplicate, but have a look at my answer to Entering a black hole, jumping into another universe---with questions. For certain types of black holes it's possible to find a trajectory that takes you inside the event horizon then back out again, but when you emerge you'll find there is no way (without travelling faster than light) to get ...

4

It's a common claim that certain types of black holes provide a gateway to a parallel universe, however there are two problems with this claim. Firstly, although it's true that trajectories can be traced through the event horizon and back out again, it isn't clear whether this is physically meaningful or just a mathematical trick. In fact if recent ...

4

Assuming no energy input, the lifetime of a black hole is related to its mass by: $$T = \frac{5120\pi G^2}{\hbar c^4}M^3$$ There is a nice summary of the derivation of the lifetime on this web site. I make the condition assuming no energy input because for large black holes the Hawking temperature is less than the temperature of the cosmic microwave ...

4

Over the years there have been suggestions that elementary particles may be black holes. However no-one has ever been able to make this quantitative and I doubt anyone believes it these days. There was some discussion of this in what is the difference between a blackhole and a point particle, and Googling will find you lots of hits on this subject. I've ...

3

The point is that the configuration that you are describing as the time-reversed Oppenheimer-Snyder collapse would need very specific initial conditions to occur. A collapse process on the other hand is very generic. You can have a collapse with any number of initial configurations. In that sense a collapse process is more physical than the reverse. It is ...

3

Dear Kahtrijn, white holes are microscopically the same objects as black holes, and it's guaranteed - by the second law of thermodynamics - that all macroscopic processes occur in the way as they do in black holes and not white holes (the latter are time-reversed of the former). However, if you don't care about the second law of thermodynamics that prevents ...

3

I don't know the answer for the case with two horizons (non-extremal case), but when there is only one (that is the extremal case: Q=M in suitable units and common notation for charge and mass), here is how it works. Let us first note that in the extremal case, the timelike coordinate outside the black hole remains timelike inside it. This is a major ...

3

The reason is that black holes are thermal equilibrium states, they emit Hawking radiation. So if you put a black hole in a box with thermal radiation, it will reach equilibrium, sometimes it will fluctuate out of existence, increasing the temperature of the radiation slightly, sometimes it will fluctuate back. The whole thing is CPT invariant. But if you ...

3

No. Courtesy of Star Trek, and numerous other Sci-Fi films and books, people tend to have the idea that anti-matter is somehow weird and mysterious. Anti-matter is just matter - it's anti only in the sense that a particle and anti-particle can react to produce two photons. The stress-energy tensor that is the source of spacetime curvature makes no ...

2

It is not necessaryly the case that black holes connect two parallel universes. But instead, that if there are parallel universes (at least one), a black hole (the sink) in one universe can connect to another universe through a wormhole (the pipe). The exit of a black hole in another universe or in another region of our universe would be a white hole (the ...

2

QFT states, that e.g. $e^+ e^-$ pairs are produced and annihilated all the time. Now is is possible, that such a pair is created at the event horizon (EH) of a black hole (BH) and one particle crosses the EH. Since the two cannot annihilate, the particle on our side of the EH lives on and gets away from the BH as Hawking radiation. However, through this ...

2

Black holes are not really as exotic as popular media makes them out to be. Modern physics does not predict them to be "gateways" to other universes. Any matter that falls into a black hole gets crushed in the singularity at the center, and will remain there. It's true that anything falling into a black hole becomes causally disconnected from the rest of ...

2

This model you are talking about, parallel universes and black holes connecting with them is a popularized version of a model that is not mainstream physics. Black holes exist because we have seen their influence, we have not seen any white hole sources of particles, this is an experimental fact. This is the reason that the black holes of mainstream ...

2

White hole is an impossible object in universe. Mathematically it is a black hole under inverted time. This can be interpreted as a black hole in an universe where second law of thermodynamics is inverted, that is the entropy always diminishes. Since second law of thermodynamics has probablistic nature, one can see a white hole as a highly unprobable ...

2

No, physically, no new Universes ever get formed. In particular, the extended Penrose causal diagrams with new infinite regions just show the maximal extension of the spacetime that is possible mathematically, ignoring physical processes inside the black hole. In physics and reality, the extension is unphysical because the naive extrapolation by Einstein's ...

2

So, I brought this up at my research group meeting this week. Turns out my initial guess was right--the past development of the Oppenheimer-Snyder spacetime does contain a white hole. When people say that the matter distribution cuts off the white hole, what they generally assume is that the spacetime does not contain a moment of time symmetry, and that ...

1

OT, but are you related to Adam Ondra, the rock climber? That guy is an animal. In most areas of physics, we're accustomed to having global symmetries such as parity and time-reversal. These are interpreted as arising from symmetries of the fixed spacetime background, which is Minkowski space. In GR, we aren't guaranteed to have any such symmetries. In ...

1

A black hole pretty much is the same as a white hole. Hawking's result proves they're essentially the same object, so the result will be a black hole with a radius larger than the sum of the radius of the black hole and the "white hole". I'm just an undergraduate so possibly one of the other members can give a more detailed answer. edit: I implied but ...

1

This question has a somewhat faulty premise that a white hole and a black hole (or anything else for that matter) could collide. In fact a white hole is defined as an area of spacetime where nothing could enter from the outside, or, mathematically speaking, it is a maximal extension of a black hole (part of an eternal black hole that wasn't formed via ...

1

A "white hole" is the exact same thing as a black hole, as discovered by Hawking and explained by 't Hooft. The reason is that black holes can be in thermal equilibrium with radiation, and the time reverse of a thermal equilibrium state is still thermal equilibrium. This intuition is confirmed in AdS/CFT, where a thermal black hole in AdS can be described by ...

1

I can't make sense of your quoted phrase "The matter distribution cuts off the white hole solution.", but I'll try to answer the rest of the question The Oppenheimer-Synder model for black hole collapse is a solution of the equations of classical GR where a uniform speherically symmetrical dust cloud with no pressure or rotation collapses to form a black ...

1

A white hole is in a sense a time reversed black hole. If you take an Eddington-Finkelstein diagram and turn it upside down you have a white hole. A physical sense of things suggests these do not exist. The argument for “cutting off” is usually interpreted as how the Penrose conformal diagram with two square patches for time like regions and two ...

1

A cosmological horizon isn't the same thing as a black hole horizon--the black hole horizon is an essential feature of the spacetime that is located where it is due to special geometry. A cosmological horizon is an observer-dependent phenomenon that describes when two observers are out of causal contact with each other. The only sense in which white hole ...

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Real black holes form through stellar collapse (and possibly a few other mechanisms) and from the point of view of an observer observing the black hole from a safe distance away they would be able to easily talk about a time before the black hole existed. When the first black hole solutions were created they used the idealization that black holes, from the ...

1

When students are taught GR one of the metrics they learn about first is the Schwarzschild metric. We tend to say, rather casually, that the Schwarzschild metric describes a non-rotating black hole. However the Schwarzschild metric is time independant, so it describes a non-rotating black hole that has existed for an infinite time and will exist an infinite ...

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I think it helps to define quite a few terms. We have spacetime, it's where we keep all our stuff both the places where they happen and the times when they happened. For instance the point $(10,0)$ could be a distance ten away from our origin, and a time of zero from our origin. And the point $(10,1)$ could be a distance ten away from our origin, and a ...

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